The invention relates to a bimetallic switch having a bimetallic element adapted to snap between a first mode and a second mode.
Temperature controlled bimetallic switches are well known in the art. The bimetal supplies the force to open or close the contact system of the switch. These switches are used to prevent overheating or overcurrent conditions in a great number of electrical appliances, both large and small, such as household appliances, automobile components, and office automation equipment.
Various shapes of bimetallic elements are available, such as discs or cantilever strips. The thermal deflection of a strip element is usually easier to predict because formulas to predict deflection due to temperature are readily available. On the other hand, bimetallic discs are also useful. These tend to change from a concave shape to a convex shape in response to changes in temperature.
There are three classifications to the type of bimetallic switches available in the art. These are: automatic reset, manual reset and one shot. Automatic resets have two distinct temperature points. A normally closed switch will open when exposed to heat at the higher of the two temperature points. When the switch cools, it will then automatically close at the lower temperature threshold. Manual reset switches, on the other hand, are typically closed, and have only an open threshold temperature. They require an outside force (such as, for example, a push button), to reset the bimetal disc after it has been opened. One shots will snap open only once and will not reset under normal conditions. They are actually automatic devices, except that the disc has been formed to have a low temperature point of xe2x88x92100xc2x0 Celsius. While one shots and manuals are typically used to protect a process, automatics are used on the other hand to control a process. There is, in addition, a hybrid version of an automatic switch, which is called a power down reset. The device includes a heat source which is activated when the contact portions are opened because of increased temperature. This heat source produces enough heat to keep the bimetal above the low reset temperature threshold. In order for the device to reset, power to the switch must be removed and the temperature be low enough for the device to reset.
Bimetallic discs are said to xe2x80x9csnapxe2x80x9d when they change state from a concave shape to a convex shape when exposed to increased temperature. The time that the disc takes to snap is related, among other things, to the temperature change that it is exposed to.
One problem associated with the use of bimetallic switches is creep. The snapping over of the disc does not occur evenly across the snap time. There is in fact a transition state where the disc moves very slowly when compared to the total action time. This slow rate of movement is called xe2x80x9ccreepxe2x80x9d. Thus, when a formed snap acting bimetallic element gets close to its snap temperature (i.e. the threshold temperature at which the bimetallic element snaps in order to open or close the switch), it begins to deflect slightly. Depending on how the contact system is arranged, this small deflection or creep may lead to a contact gap prior to a snapping over of the bimetallic element. It is to be noted that the size of the arc is additionally related to electrical load and voltage as well as to other environmental factors. As a result, some thermostats may not arc because of the loads they switch.
Among examples of bimetallic switches of the prior art which may exhibit the problem of creep are those described in U.S. Pat. Nos. 4,862,133, 4,517,541, 4,424,506, 3,577,111, 3,067,306, 2,724,753 and 2,340,056.
Various designs for bimetallic switches have been proposed to resolve the problem of creep.
Japanese Patent Number 63-292539 discloses a bimetallic switch designed to prevent malfunction and vibrations during normal service. Here, as seen in the figures of that patent, the bimetallic disc 4 is supported by support pieces 7 having supporting surfaces 6, which are formed alongside the locus of the ends 8 of disc 4 under the service temperature thereof. Even though the disc deforms within the temperature range of its service temperature, the convex side of the central part 5 of bimetal 4 does not push the base 1 or movable contact piece 3, or separate therefrom, to a great extent. In this way, malfunction due to creep, and vibrations, may be prevented.
U.S. Pat. No. 5,121,095 uses an elaborate spring member independent of the contact arms to remove creep. The switch contacts are actuated via the bimetal to a spring member, to an insulated pin, and to a contact arm system.
Other current designs remove creep through an expensive measurement and custom part assembly process which involves the installation of a pin measured to very mall tolerances, such as 0.001xe2x80x3, in each thermostat assembly. In such a case, a manufacturer must stock numerous pins to allow for the tolerance stack of the final assembly.
The above devices, which address the problem of creep, nevertheless do so at the. cost of having to provide complicated and difficult to manufacture designs for bimetallic switches. Attention must be paid to the costs of manufacture including both labor and material costs.
It is therefore an object of the invention to provide a simple and cost effective bimetallic switch which allows for a creepless contact system. It is a further object to provide a bimetallic switch having a small footprint and employing a disc which is smaller relative to discs of the prior art.
The above object, and others to become apparent as the description progresses, is achieved by the provision of a bimetallic switch comprising: a bimetallic element being adapted to snap between a first mode and a second mode, the first mode corresponding to a closed position of the switch, and the second mode corresponding to an open position of the switch; a first flexible contact member disposed adjacent the bimetallic element and having a first contact portion thereon; a first terminal electrically conductively coupled to the first flexible contact member; a second flexible contact member disposed adjacent the first flexible contact member and having a second contact portion thereon, the first contact portion and the second contact portion being disposed relative to one another such that, when the switch is in a closed position, the first contact portion and the second contact portion are biased against one another by the bimetallic element to be in engagement with one another, and when the switch is in an open position, the first contact portion and the second contact portion define an open contact gap therebetween; a second terminal electrically conductively coupled to the second contact portion such that when the switch is closed, electrical continuity exists between the first terminal and the second terminal; wherein, when the bimetallic element exhibits creep by deforming prior to a snapping thereof from its first mode into its second mode, the first flexible contact member and the second flexible contact member flex in a deformation direction of the bimetallic element such that the first contact portion and the second contact portion remain engaged until the snapping of the bimetallic element from its first mode into its second mode. Advantageously, the bimetallic element may be configured as a bimetallic disc.
According to one embodiment, the bimetallic switch comprises a housing having inner walls defining a partial enclosure therein, wherein the bimetallic element, the first flexible contact member and the second flexible contact member are disposed within the partial enclosure. Advantageously, the inner walls of the housing may define a recess therein as part of the partial enclosure, the bimetallic element being accommodated in the recess. Additionally, where the bimetallic element is a bimetallic disc, the recess may be configured to be cylindrical and to have a seat accommodating an outer edge of the disc therein. A cover may further be disposed over the partial enclosure to close the same for defining an enclosure about the bimetallic element, the first flexible contact member and the second flexible contact member. Additionally, a gapped or open cover may be used in conjunction with a thin wafer or film disposed between the cover and the partial enclosure for enhancing a thermal response of the switch. Optionally, the housing is one of parallelepiped-shaped, round and oval.
According to one embodiment of the invention, the first flexible contact member comprises a first cantilever arm; and the second flexible contact member comprises a second cantilever arm, the first contact portion and the second contact portion being disposed on respective free ends of the first cantilever arm and the second cantilever arm. The second contact portion may comprise a flange extending toward the first contact portion.
According to another embodiment, the first contact portion is positioned with respect to the second contact portion for maintaining a minimum open contact gap therebetween thereby isolating creep during contact closure.